08 yz250f quickshot 3 how to adjust? very rich bottom end.

I bought an 08 yz250f a few days ago and I am trying to get the carb dialed in. It is in very good condition, so I am very happy with the purchase as it has 20 hours on it and I picked it up for 1800. He put in a quick adjust mix screw, boyesen qs3, and no toil powerflow kit. First of all I just took the carb off, disassembled cleaned and reassembled hoping the incredibly rich boggy bottom end would have been a plugged pilot or leak jet circuit. anyway, it runs alot better, but still is very rich bottom end and will stall out if you grab some throttle. when i first took off the carb I blipped the throttle and the squirt was very very long and strong, probably much to strong for the 250. My guess is that that is the problem.


when you turn in the screw on the qs3 does that increase or decrease the squirt? I have the mix screw set at 2 and 1/4 turns out now and factory needle and clip position, a 175 main and factory pilot. any help and input will be greatly appreciated as I dont have alot of time to work on my bikes anymore. I would like to start riding more than tuning! 

The QS 3 is notoriously bad for most low rpm bog problems.  It works poorly.

Usually, all you need is the oring stretched around the apump linkage, and the proper leak jet (50) to get the job done, along with timing the squirt to miss the slide (visually).

Or even better, the R&D or Merge racing linkage spring upgrade; it won't fail like the oring.

The Apump system only effects fast throttle openings, and has no effect on the rest of the rpm or powerband.

Even if you disable the apump system, if you turn the throttle and a normal pace, you won't even notice it missing.

Your problem is probably the needle, or a worn out needle jet.

Try dropping the needle down and see if it helps.



Gently close the adjust screw on the QS3. That equates to no leak jet. At about 7/8ths of a turn open, it is the same as a #40. Set it there.


A strong squirt id good. Total duration will be under 1.5 seconds to as short as .7 seconds. You cannot time that with a stop watch, you must digitally record it then count frames. All FCR-MX have reasonable durations.



What makes you think it is rich? If it is, perhaps the fuel screw at idle is off (it should be set for a perfect idle with a hot engine)

Thanks for the quick response Guys. Well I did adjust the mixture screw this afternoon as directed on the sticky on the page here on thumper talk. And I tried running in many different positions on the leak adjust screw. But I can try and better explain this bog to you. Its consistently running this way once warmed up- I can grab a handful of throttle quick, and it doesn't bog out like it's not getting enough fuel, it just stumbles and takes a bit to clear out. I can also give it like a tiny bit of throttle (maybe 1/8) multiple times in a row and kill it. I'm convinced that the squirt duration is far too long. The screw on the throttle linkage is turned pretty far out. If I turn the screw in does that delay the squirt? How exactly does that work ?

 The screw retards/delays the squirt. You want it to just barely miss the slide every time.

You want a larger leak jet (adjustable turned in) to get 'less' squirt time.

Just turn it all the way in and start backing it out till you like it.

Wait so turning in the qs3 gives it full squirt or no squirt? I'm tempted to get a factory pump cover and a 40 Lj to see if that's really my problem.

The ap linkage looks like nobody has messed with it so I don't know.. You would not think that moving the clip position would make this much of a change in order to fix I mean this is bad! I could take a video of it and post it on YouTube or something so you can hear and see what's going on and better diagnosis

The QS 3 is an adjustable leak jet.

Turning it in means a smaller leak jet, which is MORE gas, longer squirt

You could disable it (which is a good idea) by turning it out 3 turns, to verify the actual jetting performance.

You NEED to tune the apump and add at least the oring to get any real results.

If you look at the top of the forum at the pinned posts, you will learn how.

Remember, testing the bog sitting still means nothing. You have to test it under load.

Well I warmed up the bike and rode with the qs adjusted all the way in an it was boggy. All the way out and was very boggy. Ran best at one turn out. It has to be jetting. The guy I bought it from said he re jetted it because it was way lean when he got it. So he richened it up. I was able to kill the bike by setting idle low and turning in the pilot mixture screw in, that means my pilot jet is the appropriate size? I guess next thing to do is raise the clip on the needle? If I get time tomorrow I'll try that and see how it turns out. Any thoughts on what I've done so far?

You need to set the fuel screw.

If it requires more than 2 turns to reach max idle, the pilot is too small.

1 or less, it's too big.

Yes, drop the needle (raise the clip)

In thinking about just getting a new needle an pilot jet, they don't have numbers on then so undone even now what they are. Where would be a good place to order from? Are different brands much better than others?

Use ONLY genuine Keihin carb parts.

That is what Yamaha sells.

Yamaha also has OEM speciality parts in their carbs, made by Keihin, but sold only through Yamaha

Your SERVICE MANUAL covers all this

Edited by Krannie

Had some time to work on it over the weekend. I installed the new carb kit (all jets, needle....) and it was leaking gas out the overflow. I tried it any way and it rode The same only worse. I put the old float needle back in and lowered the float to about 8-9mm and now it runs like a bike should! I guess that was the problem! Funny how it just appeared like that. I will have to do some jetting and fine tuning now. Just happy to have that terrible stumble and bog gone.

You were drowning the jets.....

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